This invention relates to a method of refining a laser gas used in a rare gas fluoride excimer laser.
As high-output ultraviolet lasers, applications of excimer lasers using a rare gas fluoride excimer such as ArF, KrF or XeF have rapidly expanded in the manufacture of semiconductor devices, in photochemical reactions and in many other fields.
A rare gas fluoride excimer laser uses a mixed gas which is comprised of a selected rare gas such as Ar, Kr or Xe, a fluorine source gas such as F.sub.2 or NF.sub.3 and an inert diluent gas such as He or Ne. The fluorine source gas is a highly reactive gas which readily reacts with surrounding materials such as the laser container materials. Therefore, it is inevitable that during operation of the excimer laser some impurity fluorine compounds are formed in the laser gas while the concentration of the fluorine source gas gradually decreases. As the laser gas is deteriorated in such a manner the output power of the excimer laser lowers considerably, so that the excimer laser cannot continuously be operated for a long period of time, if no countermeasure is taken.
In industrial applications of rare gas fluoride excimer lasers it is impermissible to simply dispose of deteriorated laser gas and replace it by a fresh laser gas since very expensive rare gases are used.
U.S. Pat. No. 4,740,982 shows a method of refining a laser gas used in a rare gas fluoride excimer laser by sequential treatments of the laser gas with selected reactive and absorptive agents. First the laser gas is brought into contact with a metal high in reactivity with fluorine, such as Si or Fe by way of example, to convert the fluorine source gas such as F.sub.2 or NF.sub.3 to a metal fluoride. Next, the laser gas is brought into contact with a solid compound of an alkali metal or an alkaline earth metal, such as Ca(OH).sub.2 or soda lime by way of example, to convert most of gaseous fluorides into solid metal fluorides. The remaining portion of the laser gas is brought into contact with zeolite to remove the remaining impurities by adsorption. The refined gas is a mixture of the principal rare gas such as Ar, Kr or Xe and another rare gas used as diluent. This rare gas mixture is returned to the excimer laser, and the laser is replenished with the fluorine source gas.
The laser gas refining process according to U.S. Pat. No. 4,740,982 can be performed during operation of the excimer laser, and this refining process is very effective for preventing lowering of the laser output power. However, even though the laser gas refining process is used the laser output power lowers considerably when the excimer laser is long operated. For example, a rare gas fluoride excimer laser can be operated to successively make 10.sup.4 shots of laser oscillation without significant lowering of the output power, but when the laser is further operated to make 10.sup.5 shots of laser oscillation in total the laser output power decreases considerably to the extent of about 50% of the initial level.